The field of this invention relates to crane structures for raising and/or lowering extremely heavy pipe lengths.
The tremendous increase in offshore oil well completions has effected the need for underwater pipelines for carrying the oil to shore or to offshore central storage facilities. Underwater pipelines for carrying oil or other fluids are generally formed with extremely large pipe lengths which may be, for example, six feet in diameter, which pipe lengths are weight-coated with concrete or other material. These underwater pipelines are laid from pipeline barges or other vessels which use various types of cranes to move the pipe lengths from storage areas on the barges to the pipe joining stations.
A davit is a type of crane which has been used on pipe laying barges and includes a boom assembly and a separate hydraulic ram for controlling the pivotal movement of the boom assembly. One such davit utilized a sheave system which included 8 sheaves to reduce the load sufficient so that a winch system could effectively operate to raise or lower the pipe lengths. One of the disadvantages of this type of davit was that the line tended to tangle in the elaborate sheave system, particularly when the davit was not loaded.
Another type of davit used a single chain line for raising and lowering such pipe lengths. However, the chain line type of davit required a separate storage bin for receiving the chain since the chain could not be wound about a drum.